class Model(object):
    def __init__(self):
        self._data = ["foo", "bar", "baz"]
        self.controllers = []

    def data(self):
        return self._data

    def add(self, value):
        self._data.append(value)
        self.changed()

    def delete(self, index):
        del self._data[index]
        self.changed()

    def changed(self):
        for controller in self.controllers:
            controller.update()
